The present invention relates to a developing unit for an electro-photographic apparatus.
A developing agent commonly used in a developing unit for electro-photographic apparatus consists of a mixture of a toner of ink powder and a carrier frictionally charging the toner. The toner density (mixing ratio of toner and carrier) of the developing agent is controlled so as to be maintained at a preset value corresponding to the specification of the electro-photographic apparatus in which the developing agent is to be used.
The toner density of the developing agent being used is different depending on the characteristic of electro-photographic apparatus. For example, the toner density of a developing agent required for developing a given electro-static latent image with a given developed density is low in a developing unit having a high developing capability. Therefore, in an electro-photographic apparatus, a developing agent is used having a toner density (for example, 3%) determined in accordance with the electric potential characteristic of the electro-static latent image formed by the latent image forming unit and the developing characteristic of the developing unit. The toner density the developing agent is decreased with consumption of toner which attaches to the electro-static latent image. In a common developing unit, the developed density decreases as the toner density of the developing agent decreases. Therefore, such a developing unit has a toner density sensor for detecting the toner density of the developing agent, a comparator for judging whether the toner density is high or low by comparing the toner density detected by the toner density sensor with a predetermined reference level, and a toner supplying unit for supplying fresh toner from a toner hopper to the developing agent corresponding to the judged result.
FIG. 7 shows characteristics of the toner supplying operation in a conventional common electro-photographic apparatus. The toner density of the developing agent detected by the toner density sensor is compared with a preset reference level, and if the toner density is decreased to a level lower than the reference level with consumption of the toner in the developing agent, new toner is supplied to keep the toner density at a certain density (for example, 3%).
As a parameter of the developing agent which is a indicator of the developed density, there is the amount of friction charge of the toner due to the carrier. In a conventional toner density control method having a target value of a fixed normal reference level, as shown in FIG. 8, the amount of friction charge of the toner excessively increases in the initial stage of loading a fresh developing agent prepared to meet the reference level as shown in FIG. 9, and the developed density is apt to decrease as shown in FIG. 10. Since the friction charge characteristic of a fresh developing agent varies depending on the condition of preparation of the toner and carrier such as the characteristic of the developing unit, the moisture control of the environment and so on, it has been difficult to stabilize the developed density of the developing unit loaded with a fresh developing agent prepared in to have a given toner density.